1. Field of the Invention
This invention relates to the gasification of black liquor. More particularly, this invention relates to an apparatus and process for controllably gasifying an aqueous black liquor by means of a molten salt to produce a combustible gas.
2. Prior Art
In the production of pulp and paper using the sodium-based sulfate and sulfite processes, digestion of wood with aqueous alkaline solutions results in the production of a byproduct which is known as spent or black liquor, hereinafter referred to as black liquor. In order to realize economies in the overall pulping process, this byproduct may not be disposed of as a waste material but instead must be converted into useful products. In particular, it is desired to regenerate sodium sulfide, which can be used to reconstitute active solutions for the pulp digestion step of the process. In addition, it is desirable to utilize the black liquor as an energy source.
The most widely practiced method of processing black liquor makes use of the Tomlinson recovery furnace (also referred to as the Tomlinson recovery boiler). In this system concentrated black liquor is burned in the furnace of a specially designed boiler to produce steam; a molten salt product generally referred to as "smelt" or "melt", which contains sodium carbonate and sodium sulfide; and non-combustible flue gas which, after suitable cleaning, is vented to the atmosphere. The process using the Tomlinson boiler has served the pulp and paper industry for about fifty years, yet it has serious deficiencies. The large volume of flue gas is difficult to clean and can constitute an environmental problem; all recovered energy is in the form of steam which has limited utility; explosions can occur if the boiler tubes leak and cause water to contact the smelt; and the reduction of sulfur compounds to sulfide is incomplete.
Various processes involing alternatives or improvements to the Tomlinson furnace have been used or proposed for converting black liquor to useful products.
U.S. Pat. No. 1,808,773 discloses a process which utilizes a black liquor recovery furnace having two zones of combustion. In the first high temperature combustion zone, black liquor sprayed into the furnace is dehydrated and substantially completely burned. In the second zone, located between the first zone and the bottom of the furnace, an additional quantity of black liquor is sprayed into the furnace along with sodium sulfate. In this second zone, water is removed from the black liquor by evaporation. Partial combustion of the black liquor results in the formation in the bottom of the furnace of a solid smelting bed of spongy carbon, mixed with alkali residues from black liquor and added sodium sulfate. Reducing conditions maintained in the bottom of the furnace result in the reduction of sulfate to sulfide. The molten salts trickle downward through the bed of spongy carbon and leave the furnace via a bottom drain. Although this process provides an alternative to use of the Tomlinson recovery boiler, the necessity for two discrete combustion zones requires a cumbersome apparatus. Also, the absence of any provision for heat recovery results in the loss of the heating value of the black liquor. Further, while there is conversion of sodium sulfate to sodium sulfide and the combustion of black liquor, the percentage of unconverted sulfate is relatively high, ranging from 8 to 12%.
U.S. Pat. No. 1,931,536 describes a process for controlling the zone of combustion of both sprayed black liquor and black liquor powder in a smelting furnace. An inert gas is introduced into the smelting furnace at or near the point of entrance of the sprayed black liquor or dried black liquor powder. This inert gas retards the combustion of the volatile constituents of the black liquor and permits the sprayed concentrated liquor or the dried black liquor powder to be projected into the smelting furnace for some distance before combustion of the organic and carbonaceous content of the black liquor occurs in a relatively deep bed in the smelting furnace. This process represents an improvement over the conventional Tomlinson recovery boiler but has the same basic limitations; the black liquor undergoes complete combustion to produce a large volume of impure flue gas, and only steam is produced.
U.S. Pat. No. 2,056,266 describes the use of a combined smelter and boiler, much like the Tomlinson boiler, for recovering alkali metal values from black liquor and utilizing the heat content thereof. Dried black liquor solids are fed to a solid fuel bed zone where they are burned in a reducing atmosphere with the result that partially burned gases rise from the fuel bed. These partially burned gases then are completely combusted by introducing a stream of air into a combustion zone above the bed. The combustion zone contains boiler tubes for the production of steam. Flue gases produced in the combustion zone are allowed to rise, and an inert gas is blown down on the fuel bed to prevent entrainment of solids in the gases rising from the fuel bed and to create a distinct line of separation between zones. Fused alkaline values are drained from the bottom of the bed. This process requires conversion of black liquor to black liquor solids prior to introduction into the fuel bed zone. In addition, the apparatus necessary for carrying out the process is complex and requires a separate means of drying black liquor.
U.S. Pat. No. 2,182,428 discloses a process for drying waste liquors by spraying the liquor to be evaporated upon the surface of a heat transfer medium such as oil, tar, pitch, asphalt or wax. Since the heat transfer medium is inert and no combustion or reduction reactions occur, the waste liquors are merely evaporated without recovering any useful product from the evaporated liquors.
U.S. Pat. Nos. 3,639,111 and 3,718,446 disclose a process for producing a clean-burning fuel by the high temperature distillation and pyrolysis of an organic material such as kraft black liquor. In order to achieve the required cracking temperatures in the pyrolysis zone, a controlled amount of an oxygen-containing gas (up to about 15% of that required for complete combustion) is introduced during the cracking operation. Because the oxygen-containing gas, pyrolyzing black liquor and product gases flow concurrently through the system and the product gas leaves at the full reaction temperature without giving up heat to incoming material, the process is thermally inefficient. Further, the requirements for both indirect heat exchange and direct combustion result in the need for relatively large complex equipment.
U.S. Pat. No. 3,916,617 describes the use of a molten salt to produce a low Btu gas from the gasification and partial oxidation of a carbonaceous material. Carbonaceous material is maintained in the molten salt zone in order to provide the desired reducing atmosphere when air is passed into this molten salt zone for partial combustion of the carbonaceous material. When air and black liquor are introduced into a molten salt reaction zone, the heat required to evaporate water in the black liquor must be supplied by combustion reactions. This results in the requirement for a high air/black liquor ratio and the production of low quality gas (typically less than 70 Btu/scf). As a result the process of this patent is primarily useful for gasification of coal and other relatively dry carbonaceous materials.
U.S. Pat. No. 4,441,959 discloses a process for recovering heat and chemical values from sent pulping liquors which utilizes a fluidized bed reaction chamber. A concentrated spent pulping liquor is combusted with air in a fluidized bed comprising a plurality of inert solid particulate materials, at least one of which is a finer particle size than another. Following combustion, the particulate materials of finer particle size are treated in an external fluidized bed heat exchanger to recover heat and to separate the finer particles from gaseous and solid products produced in the combustion. The solid products are thereafter subjected to treatment in a molten salt reducer, which results in the production of a smelt containing sodium sulfide and other salts. The gaseous products essentially comprise a noncombustible flue gas, the heat content of which is used to produce steam. The resulting cooled flue gas, following suitable purification, can be released to the atmosphere. Although this process recovers some of the heat and chemical values from spent pulping liquors, the solid combustion products are not reduced in the fluidized beds. Therefore a separate molten salt reducer is required, adding to the complexity of the process.
None of the processes previously available are therefore seen as being capable of conveniently and efficiently recovering substantially the entire energy and chemical content of black liquor as high value products.
While not considered part of the prior art, the present inventor and his associates have previously proposed other processes for the gasification of black liquor.
Thus it has been suggested that dried black liquor solids be gasified in a molten salt pool. In such a process, a combustible offgas is produced and a high level of reduction of the sulfur content of the black liquor solids to sulfide is realized. However, it is first necessary to dry the black liquor to form the black liquor solids required as feed to the molten salt pool. This increases the complexity and cost of the process.
In. U.S. patent application Ser. No. 667,937, filed Nov. 2, 1984, the present inventor has proposed a process for recovering the energy and chemical content of an aqueous black liquor by utilizing a reactor containing a drying zone located above a gasification zone. The reactor contains a bed of porous solid carbonaceous material in the gasification zone. An oxygen-containing gas is introduced into the gasification zone in a substoichiometric amount to produce partial combustion and gasification reactions sufficient to maintain the temperature at an upper surface of the bed of solid carbonaceous material in the gasification zone in the range of from about 870.degree. to 1200.degree. C. and to form a hot combustible gas which rises from the gasification zone. A concentrated black liquor containing alkali metal oxysulfur compounds is introduced into the drying zone, and the water contained therein is evaporated by contact with the hot gases rising from the gasification zone. In the drying zone there is produced a reduced-temperature product gas and dry black liquor solids which fall onto the surface of the bed in the gasification zone. The dried black liquid solids are converted into the hot combustible gas, which rises from the gasification zone, and alkali metal salts, which melt and permeate through the bed. The product gases are withdrawn from an upper portion of the drying zone. A melt in which the sulfur content is at least about 80% in the from of alkali metal sulfide is withdrawn from a lower portion of the gasification zone. Despite the advantageous features of this process in promoting gasification and sulfur reduction reactions, the reactions that occur are inefficient because of the relatively poor contact between the air and solid carbon. Also, operating characteristics are uncertain in that the bed of solid carbonaceous material can change height with minor fluctuations in operating conditions.
In U.S. patent application Ser. No. 699,498 filed Feb. 8, 1985, the present inventor has described the gasification of aqueous black liquor using a molten salt pool. An oxygen-containing gas is introduced beneath the surface of the molten salt pool, which comprises an alkali metal carbonate and an alkali metal sulfide contained within an enclosed gasifier vessel, at a rate sufficient to produce a high degree of turbulence in the molten salt pool. Black liquor in the form of a coarse spray is introduced into the rising hot gases above the pool. Thereby, water is evaporated from the aqueous black liquor into the hot gases to produce a reduced-temperature product gas and dried black liquor solids, which fall onto the surface of the pool and are dispersed therein. The dried black liquor solids are converted in the pool into a hot combustible gas, which rises out of the pool, and into alkali metal salts, which merge with the existing salts in the pool. A stream of product gas with a dry basis higher heating value (HHV) of at least about 90 Btu/scf is withdrawn from the gasifier vessel together with a molten salt product in which the sulfur content is at least about 90% in the from of alkali metal sulfide. Although the process of this invention is of utility in producing the desired results of providing a combustible gas and a molten salt product in which alkali metal sulfide predominates, the process is subject to certain problems. Corrosion and destruction of containment materials are generally inherent in the use of turbulent pools of molten salts. Also, entrainment of molten salts may occur in the gases rising out of the pool. This may require limiting the gas velocity through the pool. It has further been found that some of the carbonaceous matter in the black liquor is gasified before the articles reach the pool. As a result, only a portion of the carbonaceous matter enters the pool. If all of the air required for gasification of the black liquor is fed to the pool beneath its surface, conditions within the pool may be too strongly oxidizing for effective reduction of sulfur compounds to occur.